The present invention relates to an offset printing machine having two printing units independent of each other. More specifically, the invention relates to the offset printing machine having a first printing unit in which an ordinary offset printing is performed and a second printing unit in which a numbering unit provided with an independent impression cylinder is incorporated for number printing operation.
Various conventional offset printing machines have been known where an ordinary offset printing and numbering are achieved. In one conventional offset printing machine, a numbering device is accommodated in an impression cylinder of the printing machine to simultaneously achieve ordinary printing and numbering on the impresion cylinder. That is, the impression cylinder is commonly used for both ordinary printing and numbering operations.
Another conventional offset printing machine has been proposed where a chain delivery section incorporates therein a numbering device, so that an ordinary printing is achieved at an impression cylinder and thereafter, number printing is achieved at the chain delivery section.
In these types of the conventional machines, lateral or transverse perforations or slits cannot be formed at the ordinary printing and the numbering operation, otherwise the main impression cylinder may be damaged. Therefore, after numbering operation, the printed paper must be subjected to perforating operation by a special perforating machine. Therefore, processing period may be prolonged for finally obtaining output print image, numbering image and lateral perforations on the printing paper.
Still another conventional offset printing machine has been proposed in which additional impression cylinder is provided for numbering operation in addition to an impression cylinder for an ordinary printing operation. The two printing sections are communicated with each other by a transfer cylinder. By the provision of the additional impression cylinder, the ordinary printing is carried our independent of the numbering operation, and further, lateral perforations can be formed on the additional impression cylinder simultaneous with the numbering operation. Such conventional machine is disclosed in U.S. Pat. No. 4,426,929 assigned to Ryobi Ltd.
In this type of the machine, the printing sheet is depressed against the additional impression cylinder during the numbering operation, perforating operation and a resin relief printing operation, and therefore, the printed paper can be curved in conformance with the outer peripheral curvature of the impression cylinder. However, if such operations are not carried out, a tail edge portion of the printed paper may be separated from the cylinder surface due to warp of the paper. If such floating paper may be travelled with contacting ambient mechanical components, the printing area may be scratched by the ambient compenent to degrade printing quality. Such tendency becomes often acknowledgeable in case of the employment of a thick paper or a paper having linear orienting characteristics because of its high elasticity.